The exhaust gas emitted from an internal combustion engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that constitute particulate matter (“PM”). Catalyst compositions typically disposed on catalyst supports or substrates are provided in a diesel engine exhaust system to convert certain, or all of these exhaust constituents into non-regulated exhaust gas components.
An exhaust treatment technology, in use for high levels of particulate matter reduction, is the Diesel Particulate Filter device (“DPF”). There are several known filter structures used in DPF's that have displayed effectiveness in removing the particulate matter from the exhaust gas such as ceramic honeycomb wall flow filters, wound or packed fiber filters, open cell foams, sintered metal fibers, etc. Ceramic wall flow filters have experienced significant acceptance in automotive applications.
The filter is a physical structure for removing particulates from exhaust gas and, as a result, the accumulation of filtered particulates will have the effect of increasing the exhaust system backpressure experienced by the engine. To address backpressure increases caused by the accumulation of exhaust gas particulates, the DPF is periodically cleaned, or regenerated. Regeneration of a DPF in vehicle applications is typically automatic and is controlled by an engine or other controller based on signals generated by engine and exhaust system sensors. The regeneration event involves increasing the temperature of the DPF to levels that are often above 600° C. in order to burn the accumulated particulates.
One method of generating the temperatures required in the exhaust system for regeneration of the DPF is to deliver unburned HC to an oxidation catalyst device disposed upstream of the DPF. The HC may be delivered by injecting fuel directly into the exhaust gas system or may be achieved by “over-fueling” the engine resulting in unburned HC exiting the engine in the exhaust gas. The HC is oxidized in the oxidation catalyst device resulting in an exothermic reaction that raises the temperature of the exhaust gas. The heated exhaust gas travels downstream to the DPF and burns the particulate accumulation. A disadvantage to this method of regeneration is that the delivery of unburned HC to the engine exhaust system reduces the efficiency of the engine/vehicle since the fuel is not being used to do useful work. Additionally, depending upon the delivery location of the HC, heat loss to the engine and the exhaust system, upstream of the DPF can be significant; further reducing the system efficiency. Also, in instances where fuel is delivered by over-fueling the engine, some fuel may bypass the pistons resulting in undesirable fuel dilution of the engine oil.
Another method for generating temperatures sufficient to regenerate the DPF has involved the placement of an electric heater adjacent to the upstream face of the filter. When energized, the electric heater operates to deliver thermal energy to the upstream face of the filter that is sufficient for the ignition of the filtered particulates. A disadvantage of this regeneration method is that it requires significant electrical power to operate effectively, which is detrimental to engine/vehicle efficiency. In addition, the combustion of particulates is initiated at the leading or upstream end of the DPF filter and complete regeneration of the filter relies on propagation of the combustion throughout the entire device.
Accordingly, it is desirable to provide an apparatus and method for regenerating a DPF that will result in reduced consumption of HC and lower electrical requirements for efficient operation thereof.